Inspection of transparent substrates for defects

ABSTRACT

Methods, apparatus and systems for the detection of defects in transparent substrates such as glass sheets are disclosed. The methods, apparatus and systems are capable of detecting optical path length variations in transparent substrates smaller than 100 nm.

FIELD OF THE INVENTION

This invention relates to systems, methods and apparatus for theinspection of substrates to determine defects. The invention isparticularly useful for the measurement of one dimensional optical pathlength variations in glass sheets.

BACKGROUND OF THE INVENTION

Defects in transparent substrates such as glass sheets are typicallydetected utilizing human inspectors and manual methods. For example, inthe detection of defects such as cord and streak in glass substratesused in liquid crystal displays (LCDs), a shadow method is used todetect the defects. According to this method, a sheet of glass(typically about 1 meter wide×2 meters long) is mounted in a freelyrotating L-bracket stand and illuminated by a xenon light source. Thelight source is diverging to illuminate the entire sheet. The shadow ofthe glass is viewed on a white screen by an inspector. The defectsappear as one dimensional lines of contrast on the screen. The directionof the lines is parallel to the direction the glass sheets are drawn,for example in a downdraw apparatus in which glass sheets aremanufactured. Streak defects typically appear as a single isolated line,whereas cord defects consist of multiple lines spaced every fewmillimeters.

Cord defects typically consist of optical path length (OPL) variationsas small as a few nanometers with periods of a few millimeters. Thesesmall variations, resulting from thickness or refractive indexvariations, modulate the light intensity on the screen by an effectcommonly referred to as lensing. Repeatable and reliable visualinspection of cord and streak defects has proven to be extremelydifficult, especially using manual methods. Accordingly, it would bedesirable to provide apparatus, systems and methods capable of measuringone-dimensional optical path length variations of transparentsubstrates.

SUMMARY OF INVENTION

The invention relates to apparatus, methods and systems for measuringoptical path length variations in transparent substrates such as glasssheets. The invention can be used to measure variations in thickness intransparent substrates. Streak defects and dust can be distinguishedfrom cord more easily than in presently existing methods.

According to some embodiments of the invention, optical path lengthvariations are observed indirectly by illuminating the glass with ahighly spatially coherent beam. Interference effects that can complicateinterpretation of the contrast pattern are reduced by use of atemporally incoherent beam, i.e. one with a very short coherence length.According to certain embodiments, if the OPL variations have a periodless than or about equal to the beam size, the optical path lengthvariation causes phase distortion in the beam which can be measured,thus providing a physical effect related to the variation of the pathlength (on a scale of about 10 nm.)

In certain preferred embodiments, measurement of the phase distortion isaccomplished by spatially filtering the light, for example, by using asuperluminescent diode (SLD) light source coupled to single-mode fiber.The SLD preferably has a wavelength spectrum wide enough to provide acoherence length shorter than the optical path length through the glass,to reduce interference effects. High spatial coherence can be providedby the use of single-mode fiber. Preferably, the beam is launched fromfiber, and a lens placed one focal length away from the fiber endproduces a collimated beam. Instead of using an optical fiber, spatialfiltering may also be accomplished by transmitting the light through asmall opening, preferably less than 50 microns in diameter, andpreferably less than about 20 microns.

According to certain embodiments, the light source is directed throughthe sample of interest at normal incidence, and a positive lens is usedto partially focus the beam onto a slit. Preferably, the slit is placedin front of the focal plane of the lens. According to one aspect, phasedistortion from defects in the substrate results in small changes in theintensity profile of the beam at the slit position. A sensor placedbehind the slit detects the light intensity passing through the slit. Insome embodiments, by translating the entire optical system across theglass, changes in phase distortion in the beam are measured as contrastin the light intensity versus position data. Local variations in theoptical path length of the glass can then be inferred from this data.According to some embodiments, two spaced apart light sources are usedto allow contrast in the signal which is due to one dimensional defects(i.e. cord or streak) to be distinguished from other more localizeddefects, which can include dust, inclusions, etc. Once the lightintensity as a function of the glass position is acquired, an algorithmprocesses the data to quantify the contrast in the signal.

The present invention has wider spatial frequency response rangeresulting in higher resolution of defects on the glass surface than inprior art systems. The invention provides a repeatable measurement thatis sensitive to low level defects (e.g., less than about 100 nm,preferably less than about 50 nm and as small as about 1 nm OPLvariations). The system and apparatus are compact and can rapidlymeasure defects.

Additional advantages of the invention will be apparent from thefollowing detailed description. It is to be understood that both theforegoing general description and the following detailed description areexemplary and are intended to provide further explanation of theinvention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an apparatus for measuringdefects in transparent substrates according to one embodiment of theinvention;

FIG. 2 is a drawing of a glass sheet having a cord defect;

FIG. 3 shows a schematic of an apparatus for measuring defects intransparent substrates according to one embodiment of the invention;

FIG. 4 is a graph showing the output spectrum of a light sourceaccording to one embodiment;

FIG. 5 is a graph showing actual measured cord contrast percentageversus lens to slit distance according to one embodiment of theinvention;

FIG. 6 is a two dimensional projection of line scan data acquired from asystem after scanning a glass sheet containing cord and streak defectsaccording to one embodiment of the invention; and

FIG. 7 shows a perspective view of an apparatus for measuring defects intransparent substrates according to one embodiment of the invention.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or method steps set forth in the following description. Theinvention is capable of other embodiments and of being practiced orcarried out in various ways.

The present invention relates to apparatus, systems and methods forinspecting transparent substrates such as glass sheets. The invention isparticularly useful for detecting one dimensional OPL variations on theorder of about 100 nm or less, for example, less than about 50 nm or 10nm, and as small as about 1 nm on liquid crystal display substrates.

Referring to FIG. 1 and according to a first embodiment of theinvention, an apparatus 10 for measuring defects in or on transparentsubstrates 12 is provided. The apparatus is particularly useful formeasuring cord defects in glass substrates used in liquid crystaldisplays. As shown in FIGS. 1 and 2, the optical path length through theglass sheet, measured along the z-direction, varies with position alongthe “x” axis, resulting in a pattern of features, typically called cordor streak, oriented parallel to the draw direction or the “y” axisindicated in FIG. 2. The modulation of the optical path length along “x”is periodic with periods in the range of a about few millimeters andamplitudes in the range of about a few nanometers. Still referring toFIGS. 1 and 2, the apparatus 10 includes a first light source 14 fordirecting a first beam of light 15 at the substrate 12. Preferably, thefirst light source 14 is directed at the substrate 12 so that the firstbeam of light 15 travels in a direction substantially normal to thesheet, parallel to the “z” axis in FIG. 1. Preferably, the light source14 is configured so that it can direct light at a plurality of differentlocations across a planar surface of the substrate 12. Thus, inpreferred embodiments the light source can move in the “x” or “y”direction, or alternatively, the substrate 12 can be moved while thelight source remains stationary so that light is transmitted towards thesurface of the substrate 12 at a plurality of different locations. In apreferred embodiment, the substrate is mounted so the direction of thecord defect runs parallel to the “y” direction shown in FIGS. 1 and 2and the substrate 12 or the light source 14 moved so that the beam oflight translates across the substrate in the “x” direction.

Still referring to FIG. 1, the apparatus 10 further comprises an opticalsystem 16 including at least a first sensor 18 for detecting phasedistortion of light transmitted through or reflected from the sheet 12at different locations. The phase distortion of the light is related tothe contrast of light intensity at the different locations across theplanar surface of the substrate 12. The optical system preferablyincludes a first lens 20 between the light source 12 and the sensor 18,the first lens 20 providing the ability to selectively enhance contrastof the phase distortion received at the sensor 18. According to anotherembodiment, the sensor 18 images a virtual plane 22 a or 22 b located at(22 a) or beyond (22 b) the sensor 18. As used herein, beyond the sensormeans the plane is in direction moving away from the first light source14 in the z plane. Thus, as shown in FIG. 1, the light source 14 is onthe left, and the sensor 18 is on the right, and a virtual plane 22 blocated beyond the first sensor 18 would be to the right of the firstsensor 18.

Referring now to FIG. 3, another embodiment of an apparatus 30 formeasuring defects in a transparent substrate 32 is shown. The apparatusincludes a pair of light sources 34 a, 34 b in a spaced apartrelationship. The light sources can be spaced apart by about 20 mm to120 mm. In preferred embodiments, each light source 34 a, 34 b iscoupled to a spatial filter. In this embodiment, the spatial filteringis provided by a pair of optical fibers 35 a, 35 b. Light exits thefibers 35 a, 35 b which are spaced apart by at least the diameter of thebeams of light preferably by about 30 mm and is transmitted through thesubstrate 32. In preferred embodiments, the beams of light from thelight sources 34 a and 34 b are transmitted through a pair ofcollimating lenses 36 a, 36 b, which are disposed between the ends ofthe fibers 35 a, 35 b and the substrate 32. After the light istransmitted through the substrate 32, an optical system including a pairof lenses 38 a, 38 b, referred to hereafter as contrast enhancementlenses or contrast lenses, direct light through a pair of slits 40 a, 40b and to sensors 42 a, 42 b. The optical system is operable toselectively enhance contrast of the phase distortion received at thesensor. As in the previously described embodiment, the sensors 42 a, 42b image a virtual plane located at or beyond the sensors 42 a, 42 b.Additional details of the light source and optical system are describedbelow.

In preferred embodiments, the light source has a coherence length lessthan the optical thickness of the substrate being measured to avoidinterference effects created by reflections from the two major surfacesof the planar substrate. Preferably the light source is spatiallycoherent as well. Spatial coherence means that the phase frontperpendicular to the beam propagation direction (“z” plane in FIG. 1) isuniform across the beam. Spatial coherence can be achieved by spatiallyfiltering a light source. Spatial filtering can be achieved bytransmitting light through a small opening at least about ten timessmaller and preferably 40 times smaller than the smallest lateral defectdimension of interest, or by using a source that has been coupled tosingle mode fiber.

A particularly preferred light source is a superluminescent diode (SLD).This device provides temporally-incoherent (relatively wide wavelengthspectrum) light. FIG. 4 shows the output spectrum of a superluminescentdiode in arbitrary power units according to one embodiment of theinvention, which corresponds to a coherence length of less than about 50□m. While many types of broadband sources can provide short coherencelengths (tungsten-halogen, xenon, LED's, etc.), the SLD can be coupledvery efficiently into a single-mode fiber. This allows for efficientspatial filtering of the light source while retaining reasonable powerlevels (about 1 mW out of the fiber). Spatial filtering of more typicallight sources results in unacceptably low power. Levels of contrastcharacteristic of cord and streak defects are on the order of 1% orless. Hence, the excellent power stability of SLD sources (e.g., lessthan 0.05% over about 10 seconds) is preferred.

Preferably, the light emerging from the spatial filter is collimatedprior to being incident on the substrate. Collimation of the light beamreduces system sensitivity to sheet position and motion along thedirection of beam propagation and also simplifies interpretation of thedata collected by the sensor. In a preferred embodiment, the incidentlight beam is collimated to a diameter of about 5–20 mm (1/e² diameter).The beam size sets an upper limit on the period of optical-path-lengthvariations to which the system is sensitive. A relatively large beamsize results in a system that is sensitive to streak (with typicalwidths less than about 5 mm), while being insensitive to longer periodchanges in substrate optical path length variation.

In preferred embodiments, two light sources are used and placed in aspaced apart relationship as shown in FIG. 3. Two beam paths in a systemreduce errors in measurement resulting from dust and other contaminationon the surface of the substrate. In preferred embodiments, the beams areoffset vertically (along the y axis of the substrate or parallel to thecord/streak direction) to sample different areas of the glass surface sothat contrast common to both beams is purely due to cord or streak.Contrast differences measured by only one beam can be affected by dust,pen marks, inclusions or defects internal to the glass. The offset ofthe beams should be small enough to ensure that differences in thecord/streak features sampled by the two beams are minimal. This will beconstrained by how rapidly the cord or streak features vary along they-axis. Two beams of light could be produced by a single light sourceand a beam splitter and mirror to provide the two beams. An example of abeam-splitting device is a fiber optic coupler. However, for better longterm stability, two separate sources (one source for each beam of light)are preferred.

As noted above, a lens incorporated into the system and placed betweenthe substrate and the sensor enhances the contrast resulting from cordand streak, while also resulting in a shorter path optical system.However, factors such as vibration and alignment sensitivity limit theuse of shorter focal length lenses. For example, a 50 mm to 400 mm focallength lens provides a good combination of contrast enhancement whilemaintaining insensitivity to vibration.

The spatial frequency sensitivity of the system or apparatus may beoptimized for a particular range by adjusting the distance between thelens and the plane at which the intensity contrast is measured, definedby the location of the slit, placed between the lens and sensor, in somepreferred embodiments. An example of the frequency dependence of theoptical system measured under different configurations is shown in FIG.5. FIG. 5 shows the measured contrast obtained using a lens of focallength (f) and a ten micron slit, for two cord defects with differentperiods. As the slit is moved away from the lens (moved closer to thefocal point of the lens) the contrast for each pattern increases,reaches a peak contrast, and then rapidly decreases as the slit positionnears the focal point. The peak contrast for higher frequency cordpatterns (shorter period) occurs at a plane closer to the lens.

The circles marked A, B and C in the FIG. 5 show several possiblechoices for optimized optical system design. The circle A is a point ofleast contrast enhancement, but broadest frequency response. The circleB has the highest contrast enhancement for the lower frequency peak, butreduced contrast for the high frequency peak. The circle C is apreferred operating point. This point represents a tradeoff, wherecontrast enhancement is produced without decreasing the relativeresponse of the highest observed frequency cord.

The width of a slit placed between the lens and the sensor is preferablynarrow enough to fully resolve the highest frequency contrast features,but wide enough to pass enough light power to the sensor to maintainsignal to noise ratio. To prevent errors in measurement, preferably theslit is aligned substantially parallel to the direction of the corddefect. By substantially aligned, the slit should be aligned to withinabout 0.5 degrees of the direction of the cord.

A preferred sensor is a silicon photodiode with a minimum area of about4 mm×1 mm. Preferably, each slit is coupled directly to the surface ofthe sensor. The photodiode produces a current which is directlyproportional to the light power that passes through the slit. Thiscurrent is on the order of about 10 □A, and preferably the current isamplified by a low-noise transimpedance amplifier. However, theinvention is not limited to any particular sensor. Other suitablesensors include, but are not limited to, a charge coupled device (CCD)or other types of photosensitive sensors.

The data from the sensors is acquired and analyzed using conventionalelectronics and data processing equipment, for example, a computer.Preferably, the voltages produced by the sensors are digitized using ananalog to digital converter. The processor would categorize defects(e.g., streak versus cord and dust) and quantify the magnitude of thedefects. The data is then related and analyzed to quantify OPLvariations through the sample.

An example of data acquired from an apparatus according to oneembodiment is shown in FIG. 6, which shows measured contrast across asample at a scan rate of about 100 mm/second. FIG. 6 is a 2Drepresentation of line scan data, where the contrast has been enhanced,and cord appears as periodic lines of contrast. The y and x axes of FIG.6 correspond to the y and axes of a glass sheet described in FIG. 3. InFIG. 6, cord is evident as a period change in intensity across theimage, and a streak defect is evident as a bright, isolated line nearthe center of the image.

The overall repeatability of the system of the present invention isexcellent. Results from a repeatability and reproducibility study forsamples containing cord and streak and measured by the system of thepresent invention showed good correlation to the graded defect severity(as judged by manual inspection). Positional dependence of the systemcan be minimized by fixing the position of the sensor relative to thelight source, and translating the entire optical system across theglass. Streak detection is made possible by creating a dual beam system.Through optimization of the light source and the use of a lens, thesystem is sensitive enough to detect all known cord defects at normalincidence. In particular, detection of high frequency defects ispossible.

The system of the present invention is capable of offline inspection ina factory environment. The basic technique is also capable of the datarate required for online inspection. FIG. 7 shows an example of system100 that can be used to measure substrates 112. The system includes apair of light sources 114 and a pair of sensors 116 in a spaced apartrelationship and fixed with respect to each other. Preferably, eachlight source and sensor are spaced apart by less than about 1 meter, andpreferably less than about 0.5 meters, and more preferably, less thanabout 30 cm. The substrate 112 can be moved with respect to the lightsources and sensors, or alternatively, the light sources and sensors canbe mounted to a movable stage and translated across the sheet in thedirection shown by arrow 115. Data acquired by the sensors 116 iscommunicated to an electronics pack, which is in communication with adata processor 122. The data processor can be any conventional dataprocessor such as a personal computer or mainframe computer. Themeasurement system is relatively insensitive to glass vibration, doesnot require very stringent glass positioning, and is tolerant to dusteffects.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Forexample, response to higher spatial frequencies can be obtained byeither eliminating the lens or moving the slit closer to the lens.Higher contrast due to the defects can be obtained by moving the slitcloser to the focal point of the contrast lens. A shorter (more compact)overall optical system can be obtained by using a shorter focal lengthlens on the detection side, which may result in higher sensitivity tovibration effects. While the preferred embodiments showed collection ofdata from a light beam transmitted through a sample, the system, methodsand apparatus could utilize data collected from light beams reflectedfrom the surface of the sample. Collection of both a reflected andtransmitted signal by use of a second sensor assembly permitsindependent measurement of the surface height deviation of bothsurfaces. It is intended that the present invention cover modificationsand variations of this invention provided they come within the scope ofthe appended claims and their equivalents.

1. An apparatus for measuring one dimensional defects in transparentsubstrates comprising: a pair of light sources and a pair of sensorsmounted on a movable stage, the light sources and the sensors beingarranged in a spaced apart relationship; a substrate disposed betweenthe pair of light sources and sensors, wherein the apparatus is capableof detecting one dimensional defects having an optical path length ofless than about 50 nm; and a contrast enhancement lens disposed betweeneach sensor and the substrate and wherein the sensor detects intensityvariations related to phase distortion of light transmitted through thesubstrate and the contrast enhancement lens is operable to selectivelyenhance the contrast of the intensity resulting from phase distortion.2. The apparatus of claim 1, wherein the light source includes asuperluminescent diode.
 3. The apparatus of claim 1, wherein thesuperluminescent diode has a wavelength spectrum wide enough to providea coherence length shorter than optical path length through the glass.4. The apparatus of claim 3, wherein the apparatus is capable ofdistinguishing between dust and streak defects.
 5. The apparatus ofclaim 1, wherein the light is spatially filtered.
 6. The apparatus ofclaim 5, wherein the light is spatially filtered through an opening ofless than about 20 microns.
 7. The apparatus of claim 5, wherein lightis spatially filtered through an optical fiber.
 8. The apparatus ofclaim 1, further including a collimating lens disposed between eachlight source and the sample.
 9. The apparatus of claim 8, wherein thecollimating lenses produce a beam size of between about 5 and 20 mm. 10.The apparatus of claim 1, further including a slit disposed between eachsensor and the substrate.
 11. The apparatus of claim 10, wherein a slitis mounted on each sensor.
 12. The apparatus of claim 11, wherein thewidth of each slit is 10 microns.